Arc chamber for an ion implantation system

ABSTRACT

The present invention relates to the fabrication of materials and structures having selected mechanical, thermal and electrical properties. More particularly, the invention relates to the use of these materials and structures in ion implantation systems. Structures comprising boron material provide components for use in implanters including arc chambers with which a beam of ions is generated for implantation into a target such as a semiconductor wafer.

RELATED APPLICATIONS

This application is a continuation application of InternationalApplication No. PCT/US97/17938 filed on Oct. 3, 1997 which is acontinuation-in-part application of U.S. Ser. No. 08/725,980 filed Oct.4, 1996, now U.S. Pat. No. 5,857,889, which is a continuation-in-part ofU.S. Ser. No. 08/622,849 filed on Mar. 27, 1996, now abandoned, theteachings of the above applications being incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to methods for fabricating materials andstructures having mechanical, thermal and electrical properties suitablefor use in a wide variety of applications.

Materials fabricated from powder have in the past been fabricated usinga sintering process but do not have a sufficient density to provide aproduct having sufficient mechanical strength or thermal stability.

For example, boron has a wide variety of uses, but it is a difficultmaterial to form in a desired geometry and is also difficult to machine.Arsenic, phosphorus, antimony and boron are all used as dopants in thefabrication of semiconductor devices. These materials are selectivelyionized and implanted using an ion implantation system. These systemshave an ion source that is used to generate a beam of ionized particleswhich are directed onto a target such as a semiconductor wafer. Thesesystems are complex and expensive to fabricate, operate and maintain. Aparticular problem in the use of these ion implanters is the level ofimpurities generated during use which increases maintenance, increasesdefect density in the materials produced and reduces production yield inthe manufacture of devices.

The housing for the ion source in an implanter is often referred to asan arc chamber. Arc chambers have usually been made of graphite,molybdenum or tungsten. These materials contribute to the contaminationof the beam, and consequently, they contaminate the final product.

In one type of arc chamber electrons are emitted by a cathode, usuallyby thermionic emission, and accelerated to an anode. Some of theseelectrons have collisions with gas atoms or molecules and ionize them.Secondary electrons from these collisions can be accelerated toward theanode to energies depending on the potential distribution and thestarting point of the electron. Ions can be extracted through the anode,perpendicular to it, or through the cathode area depending upon the typeof source.

To increase the ionization efficiency of the electrons in electronbombardment ion sources, several modifications have been introduced inexisting systems. An additional small magnetic field confines electronsinside the anode and lets them spiral along the magnetic field lines,multiplying on their way to the anode and increasing the ionizationefficiency of the ion source. By using a cylindrical anode and areflector electrode, the electron path is further enlarged. Many massseparator ion sources are this type, such as the Nier, Bernas, Nielsen,Freeman, Cusp and other sources.

The Bernas ion source, for example, has a rectangular or cylindrical arcchamber positioned in an external magnetic field. The source can containa single-turn helical filament (cathode) at one side of the arc chamberand a reflector at the other end. Electrons from the cathode areconfined inside the anode cylinder by the magnetic field and canoscillate between the filament and the reflector resulting in a highionization efficiency. Ions are extracted perpendicular to the anodeaxis through a slit of about 2 mm width and about 40 mm length. However,the dimensions can vary, depending on the specific design.

A continuing need exists for improvements in the field of materialsfabrication to provide structures having desired mechanical, thermal andelectrical properties. In particular, there is a need for improvementsin ion implantation systems used for the fabrication of semiconductordevices.

SUMMARY OF THE INVENTION

The present invention relates to devices and methods of fabricatingcomponents for use in ion implantation systems. More particularly, theinvention relates to the fabrication of boron arc chambers and otherboron components for ion implantation systems. With the use of boroncomponents in ion implantation systems a number of advantages arerealized, including a reduction in contaminants due to the use of boroninstead of other materials such as graphite, molybdenum or tungsten; theenhancement of beam current that can be accommodated due to the lowerlevel of contaminants; the lighter weight of these components and theability to retrofit them onto existing systems as well as their use innew systems, and the ability to use these components with the electricalsystem (e.g. as electrodes) and as a source of boron particles forionization.

The refractory metals are problematic for the ion source because theyare heavy, difficult to fabricate, and highly reactive with borontrifluoride, a gas used in many systems to provide a source of boron forionization.

Tungsten, for example, is the current preferred material for the Bernasion source but is far from ideal. It is one of the heaviest of allengineering materials having a density of 19.3 gm/cc, is difficult andexpensive to machine, and reacts with boron trifluoride to form anothergas, tungsten hexafluoride. The chemical reaction between fluorine andtungsten not only erodes the interior of the arc chamber but also actsas a material transport mechanism for depositing tungsten metal at otherregions of the chamber. This effect shortens the chamber lifetime andconsiderably alters its interior geometry. Additionally, tungstenhexafluoride formation acts to pump unwanted tungsten ions into theboron beam current, some of which invariably ends up in the target,which is typically a single crystal silicon wafer orsilicon-on-insulator (SOI) structure used for the manufacture ofintegrated circuits.

Boron is very light, having a density of 2.46 gm/cc (about 13% theweight of tungsten) and therefore, is less demanding on mountingfixtures and is easier to handle. It is also very hard and strong, evenat the elevated operating temperatures of an arc chamber. It is moredurable than graphite and tungsten, which is prone to creep (permanentdisplacement under an applied load). A boron arc chamber enhances thesource beam current by reaction with free fluorine ions in applicationsinvolving the use of boron trifluoride as a source for boron ions.

Solid boron has not been utilized in semiconductor processing systemsbecause it is not a conventional engineering material. There arecurrently no known manufacturers of dense boron products, mainly becausespecialized materials techniques are required to form this type ofboron.

Structures made from boron for use in the fabrication of implantercomponents can be made using several distinct processes. A preferredembodiment of a method for making such boron structures includesproviding a mold or die having the desired shape for the part to befabricated, positioning a boron material such as an amorphous boronpowder into the mold, treating the boron powder under selectedconditions of temperature and pressure to crystallize the powder into amore crystalline state to form a solid unitary boron structure, removingthe structure from the mold and machining the structure as necessary. Inmany applications it is desirable to produce a structure having apolycrystalline lattice with an average crystal size in the range of 1to 10 microns. In some applications it is desirable to form a structurehaving a crystal size in excess of ten microns, including single crystalmaterial. In some applications with lower tolerance requirements theremay remain a large population of crystals with diameters of less than 1micron, typically in the range of 0.5-1 micron.

It is also preferred that the density of the material produced be atleast 50% of its maximum (theoretical) density, and preferably in therange of 80-100% in order to increase the mechanical strength andresistance to erosion. A preferred embodiment employs boron having ahigh purity level having an atomic percentage of elemental boron of atleast 95%, and preferably of at least 99.99% or greater.

The fabrication process can be pressure sintering methods such asuniaxial hot pressing and hot isostatic pressing, or a casting method, asingle crystal growth method, by deposition from the vapor or liquidphase, or by spray forming. The specific technique employed for a givenworkpart will depend upon the requirements for purity, density, andgeometry as well as the mechanical, electrical, optical and/or chemicalcharacteristics desired.

For certain ion sources, a preferred embodiment of the inventionutilizes boron material as a source of boron to be ionized. Atsufficiently low pressure and high temperature boron sublimes at a ratesufficient to produce a flow of gaseous boron suitable to generate anion beam for implantation under electron bombardment.

In another preferred embodiment, boron is used as a filament to generateelectrons by thermionic emission which are then accelerated by anelectric field to bombard the boron within the arc chamber to generatethe ionized beam. A magnetic field is used to confine ions within thechamber until they are extracted through the exit aperture of thechamber to form the ion beam. When highly pure boron is heated to asufficient temperature, it becomes highly conductive. Many arc chambers,for example, operate at temperatures at which boron is conductive andcan thus employ boron as electrically conductive components.Alternatively, doped boron structures can be used which are conductiveat lower temperatures, including room temperature. Boron is also highlytransmissive in the infrared region (e.g. 1-8 microns wavelength) of theelectromagnetic spectrum and can be used as an optical window or lens. Aboron window or lens can thus be used with an infrared sensitive camerasuch as a charge coupled device to monitor thermal processes or otherinfrared imaging applications such as infrared radar.

Other components within the implanter that are exposed to the ion beam,or which are likely to contaminate the beam, can optionally also be madeof boron. These can include, without limitation, the extractionelectrode or grid, components of the beam analyzer such as the beam traptarget, beam deflectors, and components of the implantation chamberincluding trays for holding wafers, electrostatic clamps, and robot armsto control movement of objects within the implantation chamber.

The processes described herein can also be used in the manufacture ofdense boron coatings, sputtering targets, for the preparation of boroncoatings for diffusion into other substrates including semiconductors,and as diffusion furnace components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of an ion implanter embodying theinvention.

FIG. 1B illustrates an implantation system providing ion beam trap.

FIGS. 2A and 2B are cross-sectional top and side views, respectively, ofan arc chamber in accordance with the invention.

FIGS. 2C, 2D, 2E are detailed front, bottom and side views of a boronfilament in accordance with the invention.

FIG. 3A is a cross-sectional view of a multicusp ion source inaccordance with the invention.

FIGS. 3B and 3C are top and cross-sectional views of an anode ring for amulticusp ion source embodying the inventions.

FIG. 4 is a schematic view of a Freeman source in accordance with theinvention.

FIG. 5A is a process flow diagram illustrating a hot pressing techniquefor fabricating a boron structure in accordance with the invention.

FIG. 5B graphically illustrates a pre-compacting process of boronpowder.

FIG. 5C graphically illustrates a preferred apparatus for conducting hotpress under vacuum.

FIG. 5D is a graphical representation of a process for making a boronstructure in accordance with the invention.

FIG. 6 is a process flow diagram illustrating steps in an isostaticpressing method for the fabrication of components in accordance with theinvention.

FIG. 7 is a process flow sequence illustrating the steps in a sinteringmethod for fabricating components in accordance with the invention.

FIG. 8 is a process flow sequence illustrating the steps in a castingmethod for fabricating components in accordance with the invention.

FIG. 9A is a process flow sequence illustrating a method of fabricatingsingle crystal boron components for an ion implanter by pulling from amelt.

FIGS. 9B and 9C graphically illustrate the single crystal growth method.

FIG. 10 illustrates a method of using an ion implanter in accordancewith a preferred embodiment of the invention.

FIG. 11 is a schematic illustration of a method of fabricating boroncomponents of a processing chamber by deposition from the vapor or gasphase.

FIGS. 12A and 12B are top and cross sectional views, respectively, of anion source housing fabricated in accordance with a preferred embodimentof the invention.

FIGS. 13A and 13B illustrate top and cross-sectional views,respectively, of a portion of an ion source housing fabricated inaccordance with a preferred embodiment of the invention.

FIG. 14 is a cross-sectional view of a boron deposited electrodefabricated in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is illustrated generally in theion implantation 10 system of FIG. 1A. The system 10 typically includesan ion source 16, a power supply 14, an extracting electrode or grid 18,a magnetic beam analyzer 20 and a controller 22 which are positionedwithin an inner housing 12. Ions are generated at the source 16,extracted from the source 16 with electrode 18 and conveyed along theion beam path 15 within the beam housing 24 into the implantationchamber 34. The beam analyzer 20 selectively separates components of thebeam exiting from the source 16 so that only ions of the desired massare directed towards the ion implant chamber 34.

The source 16 includes an arc chamber housing 25 constructed from acrystalline boron material. There are several types of sources havingdifferent configurations of the components, however, these componentscan include an anode, a cathode, a third electrode or filament, variousports to introduce materials into the arc chamber 25 to be ionized, andan exit aperture through which ions are extracted from the chamber 25.The boron material used in the chamber will vary depending upon theapplication, but the material will usually have a uniform density ofeither a polycrystalline material or a single crystal material.

Other components of the implanter can include a beam controller 26 thatcan be used to selectively deflect the beam so that the beam can bescanned across a target 28. The implantation chamber 34 includes asupport 30 for one or more targets or wafers mounted within the chamber34 and a drive mechanism to control movement of the support 30 relativeto the implantation chamber housing 34. The system can be used, forexample in the fabrication of many size wafers from 50 mm up to 300 mmor higher. A robot arm can be used to move targets within the chamberand/or insert or remove targets from the chamber. As described ingreater detail, many components of processing systems can be made usingboron material as well as the arc chamber in order to lower impuritylevels and improve system durability and performance.

FIG. 1B illustrates another preferred embodiment of an implantationsystem providing beam trap. Similar to the system in FIG. 1A, theimplantation system 400 includes a ion beam source 402 which generatesion beam 407 extracted with the help of extracting electrodes 414 andpasses the beam through an enclosed path 404. The beam is filtered by amagnetic beam analyzer which guides a portion 408 of the source beamhaving appropriate mass. The portion 406 having heavier mass is unableto make the turn into the target guide 426 and is directed to a trap410. The trap includes a beam target or collector 428 and a valve 412 toallow removal of trap debris. The magnetic analyzer 418 is controlled bya mass controller 416. The desired ion beam 408 is guided to strike atarget 422 which can be mounted on a rotatable disk 420 driven by amotor 424.

A preferred embodiment of an arc chamber of an ion source is of Bernastype and an example is illustrated in FIGS. 2A and 2B, respectively. Thehousing 50 comprises a boron material made in accordance with themethods defined in greater detail below. The boron material ispreferably a polycrystalline material with the average grain size beingin the range of 1 to 10 microns. The density of the material is at least50% of the maximum theoretical density (TD), and preferably greater than60% TD for machining purposes. The boron housing uses boron having anatomic percentage of at least 95% elemental boron and preferably atleast 99.99%.

Other preferred embodiments of the housing material can include singlecrystal boron, or alternatively, certain components can be made withboron compounds such as silicon hexaboride, tungsten boride, boronnitride or titanium diboride which are compatible for use in combinationwith the solid boron structures described herein. The inner surfaces ofthe chamber can also be coated with pure dense boron or these compoundmaterials, with the specific type of material chosen being dependentupon the type of ion source, the desired insulating characteristics andthe gases used within the chamber.

The components within the chamber can either be the standard materials,or more preferably, these can also be formed from the same boronmaterial as the housing 50. These can include the shields 52, thecathode which serves as a source of electrons, an anode 67 and therepeller 54.

The cathode filament 58 serves as a source of electrons that bombards aboron material entering through the port 56 in the bottom wall 55 of thechamber 50 to form boron ions. FIGS. 2C, 2D and 2E illustrate front,bottom and side views of a boron filament 58. the filament 58 caninclude three elements, two posts 73, 75 and cross member 71. Member 71fits within a groove 79 in one end of both posts, at the opposite end ofboth posts 73, 75 are connectors 77 that extend through the wall of thechamber and connect to pin connectors to attach the filament 58 to apower source.

A portion of the chamber serves as the ion source anode 67. The cathode58 can be made of Ta or W. The gas flow is small so the source willoperate in a vacuum environment within the arc chamber of about 5×10⁻⁴torr. Alternatively, as described in greater detail below, the filament,the ion source and/or the cathode can also be boron material.

The top wall 51 of the chamber 50 has an aperture 62 or rectangular slitthrough which ions within a containment region 65 within the chamber aredrawn.

The filament 58 generates electrons which oscillate between the filamentand the reflector. The electrons impact particles containing boron whichhave been directed into the electron path to produce boron ions. Theions are extracted perpendicular to the anode axis through the aperture62. Additional ports 69 can be used to introduce gases into the chamber.The boron source 60 can either be a standard gas or a boron materialthat is heated above its sublimation temperature to produce a sufficientflow of boron particles.

The top 51, bottom 55, sides 53,57, plates 52, and end plates can bemade separately as described hereinafter and bonded at the edges, oralternatively, they can be mounted using an external fixture, or a largenumber of plates and rings can be made separately and pressed together.Alternatively, chamber components can be formed to near net shapewithout the need for extensive further machining.

In the multicusp ion source, the primary ionizing electrons are normallyemitted from tungsten-filament cathodes. The source chamber walls or aring form the anode for the discharge. The surface magnetic fieldgenerated by rows of permanent magnets, typically of samarium-cobalt orneodymium-iron, can confine the primary ionizing electrons veryefficiently. As a result, the arc and gas efficiencies of these sourcesare high.

The multicusp ion source is relatively simple to operate. There are fourmain components in the source: the filaments (cathode), the chamber, ananode ring, and the first, or plasma, electrode. A schematic diagram ofthe multicusp source is shown in FIG. 3A. The source chamber can berectangular, square or circular in cross section and is formed with aboron material. The permanent magnets can be arranged in rows parallelto the beam axis. Alternatively, they can be arranged in the form ofrings perpendicular to the beam axis. The back plate also contains rowsof the same permanent magnets. Grooves are generally milled on theexternal wall so that the magnets are mounted within approximately 3 mmfrom the vacuum. These magnets generate multicusp magnetic fields forprimary electron as well as for plasma confinement.

The open end of the source chamber is closed by a set of extractionelectrodes which can be made with a conductive boron material. Thesource can be operated with the first electrode electrically floating orconnected to the negative terminal of the cathode. The background gas isintroduced into the source chamber through a needle or a pulsed valve.The plasma is produced by primary ionizing electrons emitted from one ormore boron or tungsten filaments, which are normally biased negativelywith respect to the chamber wall or the anode ring. These filaments arelocated in the field-free region of the ion source chamber and they aremounted on molybdenum holders. The plasma density in the source, and,therefore, the extracted beam current, depends on the magnet geometries,the discharge voltage and current, the biasing voltage on the firstextraction electrode, and the length of the source chamber. The sourcechamber is normally pumped down to about 10⁻⁶ to 10⁻⁷ torr before gasesare introduced for beam generation.

The multicusp source for ion implantation and for surface modificationpurposes, beams of B⁺, P⁺, As⁺, and N⁺ ions have been extracted frommulticusp ion sources. By operating the source at higher dischargevoltages, ions with multiply charged states have been generated. FIGS.3A and 3B illustrate top and side cross-sectional views, respectively,of a boron anode ring for a multicusp ion source. As the anode ringoperates at a temperature of about 600° C., the anode is conductive.

Freeman developed another type of source by putting the cathode of aNier type mass separator ion source inside the hollow anode, as in amagnetron, but left the magnetic field low. The performance of theFreeman ion source has made it successful for ion implantation andindustrial application, especially for semiconductor implantation.

The Freeman type ion source as shown in FIG. 4 has a similar design to amagnetron, but it uses just a low external magnetic field of about 100G. The arc current is 1 to 3 A and the arc voltage is between 40 and 70V. A rigid cathode rod 80, usually 2 mm in diameter and made of tantalumor tungsten, is heated with about 130 A and a few volts to the righttemperature. The cathode 80, as well as the chamber 82 can be made ofboron. The axial position of the cathode in the Freeman ion source showsseveral advantages including:

1. The inherent magnetic field of the cathode forces the primaryelectrons to move around the cathode, concentrating the electron densityin this area.

2. The high electron density next to the extraction slit produces a highion density in this area, and, thus, a high ion beam current.

3. The straight-filament rod fixes the plasma parallel to the magneticfield lines and the extraction aperture, which is the reason for theexcellent beam quality of Freeman ion sources.

4. The low magnetic field does not force instabilities like the highfield in a magnetron.

The lifetime of the source is dependent upon the type of gas used orcorrosion rate of the boron or other components in the source. Changingthe polarity of the filament after some time of operation improvescathode lifetime. Heating by AC has the same effect but increases plasmainstabilities and the energy spread of the extracted ions.

The Freeman ion source is especially designed to deliver ion beams fromnongaseous materials. There are many versions with ovens for varioustemperatures and for the application of chemical compounds and in situchemical synthesis of the required material. With chemical compounds,corrosion problems occur not only at the cathode, but also at the anodeand with the oven.

Ion currents of several milliamps can be produced for most elements andmore than 20 Ma for a few elements like arsenic and phosphorus underfavorable circumstances and using large extraction areas. The extractionslit is usually about 2 mm wide and about 40 mm long. Larger slits arepossible, such as 90 mm, but there are some disadvantages because thecurrent density is not uniform along the long slit due to the biggervoltage drop along the cathode. The anode and the magnetic field can beadjusted, however, to overcome this problem.

The ion current density is controlled by the arc current, which iscontrolled by the filament, the gas pressure, and the magnetic field.The ion source is mounted inside the vacuum chamber. The feed-throughscan be mounted on a base flange perpendicular to the source axis. Theextraction slit is usually 40×2 mm but designs up to 100×5 mm have beenrealized. This type of source can use a W, Ta or boron filament, 1.5-2.5mm φ; alumina or boron nitride insulators; and operates in a vacuum atless than 7.5×10⁻⁶ torr.

FIG. 5A describes the process flow sequence of a preferred hot pressingmethod. The process begins at 200 by preparing boron of 99.9% purity indry powder form. FIG. 5B illustrates the apparatus for thepre-compacting sequence. A first die or mold 306 of desired form isfabricated at 202 to contain the boron powder. The dry boron powder isthen pre-compacted at 204 in the first die by applying ram force with apunch 302 at room temperature. The compacted boron material 304 is thenreleased upward by a static ram 308.

The pre-compacted boron powder from the first die is moved to a secondgraphite die at 206 and placed in a vacuum hot press system 320 (FIG.5C). The second die 324 is lined with tantalum foil 310 of about 0.005to 0.02 inches of thickness and boron nitride powder 312 is laid betweenthe tantalum foil and the boron powder 206. The tantalum and boronnitride liners function as barriers to prevent the formation of B₄ C.The boron material is heated at 208 under vacuum at the rate of about 50to 300° C./hr to about 1850° C. At this temperature, pressure is appliedas graphically illustrated in FIG. 5D.

The system 320 provides a cylindrical vacuum compartment 321 in whichthe boron material 304 in the second graphite die 324 is pressed by aram 322 and ram 326. The compartment provides ports for receiving argonpressure gas 330 and outlet for vacuum pump 334. The compartment furtherfeatures a pressure release port and a sealed door 336 with heatingelements 337. The heat is applied slowly to remove any impurities in theboron material and also to effect complete transformation of boron fromthe amorphous to the crystalline state. At 1850° C. the material is heldunder vacuum for 1 hour and ram pressure of about 4000 to 6000 psi isapplied at 210 over the boron material and sustained for about 2 hours.When ram pressure is applied, an argon overpressure of 300-1000 Torr ismaintained for the duration of the process. At 214, the material is thenallowed to cool, still under the argon atmosphere, at the rate of about150° C./hr. At 216, the cooled and hardened block of boron is machinedand polished to a desired configuration.

FIG. 6 describes a preferred hot isostatic compacting method for formingdense boron material. Again, pure boron powder, preferably of 99.9%purity, is prepared at 218. The powder is typically wrapped in aflexible mold material, such as silicone rubber, and compacted at 220 atroom temperature using isostatic pressure, defined as fluid pressurewhich provides uniform pressure from all direction. The pre-compactedboron is then encapsulated at 222 with a protective foil such astitanium, tantalum or molybdenum. The boron powder is then heated at 224in a high pressure chamber to about 1500° C. to 2000° C. and sustainedfor about 1 to 3 hours. At 226, an argon pressure of at least 20000 psiis applied over the boron material. At 227, the material is allowed tocool at a controlled rate of about 300° C./hr to prevent any thermalshock to the material. At 228, the boron structure is machined andpolished to a desired configuration.

Sintering is yet another method of preparing solid boron structure andis described in FIG. 7. Here again pure boron of 99.9% purity in drypowder form is prepared at 230. The boron powder is isostaticallypre-compacted at 232 using fluid pressure at room temperature, similarto the pre-compacting process described in FIG. 6. The pre-compactedboron is placed in a graphite container at 234 which is lined withtantalum foil and a layer of boron nitride powder. At 236, the containeris placed in a vacuum furnace and heated to about 1950° under vacuum atthe rate of 150° C./hr to about 400° C./hr. At 238 to 240, while theheat is maintained at this level for about 2 to 3 hours, 1 to 2 psig ofargon pressure is applied. At 242, the heat level is raised slowly toabout 2000-2100° C. and maintained for about 1 to 3 hours, still underpressure. At 244, again boron is allowed to cool at a controlled rate ofabout 300° C./hr to prevent any thermal shock to the material. At 246,the resulting boron structure is machined and polished to a desiredconfiguration.

FIG. 8 describes the method of melting to form solid boron. Again, at254 a suitable boron powder is provided. A xylene fluid is added to thepowder and the mixture is ball milled to achieve a slip of desiredconsistency. A plaster mold is fabricated at 256 to hold the boron slipin a desired shape. Prior to placing the boron powder, the mold istreated with alginate for about 60 seconds and drained at 257. At 258,the mold is air-dried for about 2 hours before being treated with xylenefor about 15 minutes at 259. The boron slip is then placed in the moldfor casting at 260. The cast boron material is removed from the mold andat 261, the boron material is heated in a boron nitride crucible undervacuum at the rate of about 300° C./hr to about 1950° C. At 262, argonpressure of 1 to 2 psig is applied over the material. At 263, thematerial is further heated to about 2200° C. and held at such a heatlevel for about 0.5 hour. At 264, the material is allowed to cool at therate of 150° C./hr to room temperature. The dense boron structure isthen machined and polished to a desired configuration at 265.

Another preferred method of making boron components for ion implantationsystems utilizes a process of pulling a boule or ingot of single crystalmaterial from a melt. This process is described in FIG. 9A andschematically illustrated in FIGS. 9B and 9C. The process includesmelting boron 272 in a boron nitride crucible, pulling the singlecrystal material from the melt at 274 at a rate and temperaturesufficient to provide a desired diameter, cutting at 276 the singlecrystal material to a desired shape. The parts are then machined andpolished at 278 to form discrete components of an arc chamber or othercomponents of an implanter, and then assembled at 280 and 282 asnecessary.

The system for fabricating a single crystal material uses a crucible 360in an oven 356 in which heaters 366 melt the boron powder to form afluid 362. A rod 350 with a seed mounted on tip 352 contacts the surfaceat 364 and is drawn up at selected speed and temperature such that boule368 is formed.

Single crystal boron can also be formed using known methods bydeposition from the vapor phase in order to form a layer of boron on anexisting part. This procedure can be used in forming filaments or incoating the internal surfaces of arc chamber or other processingchambers as described herein.

FIG. 10 describes the ion implantation process using the preferred boronmaterials of the present invention. A first boron filament issufficiently heated at 284 to produce boron particles inside anion-source chamber. A sufficient voltage is then applied at 286 to asecond boron filament to generate electrons inside the chamber to reactwith the boron particles. A magnetic field is generated at 288 by asource to contain such electrochemical reaction inside the chamber. Arepeller of opposite charge is provided at 290 inside the chamber tofurther contain the ions generated by the electro-chemical reaction. Theboron ions, thus formed, are extracted from the chamber by providing asufficiently small exit aperture on the chamber and extractingelectrodes near the aperture. The ion beam is then mass filtered at 294and direction-controlled by a second magnetic field in a sealed beampath at 296. The ion beam is then directed to a target for implantationat 298.

Machining dense boron materials is generally performed using diamondtooling including core drills, milling machines and grinding wheels.Typically cutting boron material is computer numerically controlled(CNC) for precision. Other methods include lapping, electrical dischargemachining, laser machining, grinding, and ultrasonic machining.Polishing dense boron materials typically utilizes a diamond grit paste.

Another preferred embodiment of a method for fabricating components foran ion implanter, including a boron ion source, is to use chemical vapordeposition (CVD) of boron on a substrate. In this technique, a boroncontaining gas is caused to precipitate elemental boron, either byreaction with another gas or by thermal decomposition, onto a suitablyprepared substrate. The process is schematically illustrated in theprocess sequence of FIG. 11.

The first step is to prepare a substrate 518 of high density such as asmall grained graphite or other material, preferably with a coefficientof thermal expansion approximating that of boron. The substrate can alsobe boron nitride, aluminum oxide, molybdenum, tungsten, tin, siliconcarbide or other materials that can be machined so that when a depositof boron has been made on its surfaces, it is the desired dimension.

A standard CVD reactor is configured so that the substrate can be heated520 in the presence of the boron containing gas. Heating methods can beresistive heating, induction heating, or infrared heating. Followingproper preparation, positioning and heating of the substrate a boroncontaining gas such as boron trichloride or diborane is introduced 522to the reactor and boron is caused to deposit 524 onto the hot substrateby, for example, reaction of the boron trichloride with hydrogen, oralternatively, by thermal decomposition of the diborane.

The deposition temperature at which the boron is deposited is preferablyheld below about 1500° C. so that the material remains in the amorphousstate rather than the crystalline state. Applications in whichcrystalline materials are subjected to large thermal and/or mechanicalstresses can also result in fracture of these materials along weakerplanes within the material. This problem is eliminated by the use ofdeposited amorphous boron films. The temperature is also preferably heldabove the maximum operating temperature of the component such as an ionsource to provide the desired thermal stability in the resulting film.Thermal deposition from diborone can be conducted at about 900° C. Notethat for applications requiring the use of conductive boron films, itcan remain advantageous to use polycrystalline boron to improveconductivity. Both amorphous and polycrystalline films can also be dopedto enhance conductivity. Dopants such as silicon or carbon can bediffused or implanted during or after boron film deposition.

In one example, a plate of graphite having a coefficient of thermalexpansion of 8.2 m/m/⁰ K was fitted with two electrode clamps so that acurrent could be nm through it for resistive heating. The plate andelectrodes were then placed in a reactor tube fitted with end flanges sothat it could be evacuated and then filled with flowing gases atregulated flow rates. An infrared transducer was aimed at the surface ofthe plate for providing feedback to a temperature controller/powersupply circuit. When the graphite reached a temperature of 1240° C., theboron trichloride and hydrogen gases were introduced in the ratio of 2moles boron trichloride to 3 moles of hydrogen. After one half hours ofoperation, a uniformly thick layer measuring 0.5 mm had been depositedon the graphite plate. For preferred embodiments, it is desirable todeposit films having a thickness in the range of 0.5-3 mm.

The substrate and boron film is then removed 526 from the chamber.

Depending upon the particular application or component the film canoptionally be further processed by one or more methods includinggrinding or machining of one or more surfaces or regions of the film orthe substrate to achieve desired tolerances. The film can also beseparated or cleaved from the substrate.

After any optional processing one or more of the components of theprocessing chamber can be assembled 527 and installed in the ionimplanter or other device.

Illustrated in FIGS. 12A and 12B are top and cross-sectional views,respectively, of an ion source housing 550 fabricated in accordance witha preferred embodiment of the invention. The chamber 550 is similar indimensions to that illustrated in connection with FIG. 2A, however, asshown along section A--A in FIG. 12B, the chamber includes an innershell or substrate 556 on which a thin film of boron 554 has beendeposited. Deposition can be used to uniformly coat the entire surfaceincluding ports 552, or alternatively, can cover all or selectedportions of internal surfaces 558.

Shown in the top and cross-sectional views of FIGS. 13A and 13B,respectively, is the top 570 of the chamber 550. The aperture 572 canhave a beveled shape at 578, as seen in the section B--B of FIG. 13B. Athin film of boron 576 has been deposited on substrate 574 to providethe ion source aperture.

In another preferred embodiment, the deposition process can be used inthe fabrication of other components described previously herein. Forexample, a filament or wire used in the ion source can be coated with athin film of boron to provide an electrode or boron source for use inthe ion source housing. Such a film is shown in the cross-sectional viewof FIG. 14 in which a metal filament 600 has a thin film 602 of boronthereon.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

I claim:
 1. An ion implantation system comprising:a gaseous boron sourceattached to an arc chamber of a Bernas ion source, the gaseous boronsource providing boron ions within the arc chamber; a magnetic fieldsource; an implant chamber in which an implantation target ispositioned; a scanning system that controls relative movement betweenthe implantation target and a beam of the ions; the arc chamberincluding a boron housing element having an aperture such that the beamof ions is directed through the aperture to the implant chamber, acathode within the boron housing element that provides electrons, theboron housing element comprising a boron material with a density in arange between 80% and 100% of a maximum density of pure boron.
 2. Thesystem of claim 1 wherein the boron housing element further comprises arectangular arc chamber in which the ion source is positioned.
 3. Thesystem of claim 1 wherein the ion source comprises boron.
 4. The systemof claim 1 further comprising a vacuum port.
 5. The system of claim 1wherein the arc chamber further comprises a fluid inlet port.
 6. Thesystem of claim 1 further comprising a magnet attached to the arcchamber.
 7. A method of implanting ions comprising the stepsof:providing a Bernas ion source to introduce a gaseous boron materialinto a boron arc chamber, the boron arc chamber comprising a boronmaterial with a density in a range between 80% and 100% of a maximumdensity of pure boron; generating a beam of ions within the boron arcchamber; directing the beam of ions along a beam path with an electricfield; and implanting the beam ions into a target.
 8. The method ofclaim 7 further comprising directing ions along the beam path through anaperture in the chamber.
 9. The method of claim 7 further comprisingscanning the beam relative to a surface of the target.
 10. The method ofclaim 7 further comprising providing a target including a semiconductorwafer.
 11. The method of claim 7 further comprising providing a boronmaterial having an atomic percentage of elemental boron of at least 95%.12. The method of claim 7 further comprising heating a boron electrodein the chamber to conduct a current through the boron electrode.
 13. Themethod of claim 12 further comprising conducting a current of at least 1amp through the boron material.
 14. An ion implantation systemcomprising:a gaseous boron source that provides a source of boron ions;a magnetic field source; an implant chamber in which an implantationtarget is positioned; and a Bernas ion source including a boron anodearc chamber housing, a cathode to provide electrons, a port for entry ofthe gaseous boron material, and an aperture in the boron arc chamberhousing through which boron ions are delivered to the implant chamber,the boron arc chamber housing comprising a boron material with a densityin a range between 80% and 100% of a maximum density of pure boron. 15.The system of claim 14 wherein the boron material comprises at least 95%of a maximum density of pure boron.
 16. The system of claim 14 furthercomprising a target assembly that supports a semiconductor wafer. 17.The system of claim 14 wherein the boron material has a purity of atleast 95%.
 18. The system of claim 14 further comprising a beam trap.19. The system of claim 14 wherein the boron material is amorphous. 20.The system of claim 14 wherein the boron material is attached to asubstrate.